1. Field of the Invention
The invention relates to steelmaking, particularly to additive compositions for use during the steelmaking process.
2. Description of the Prior Art
The basic steelmaking process comprises a well-known series of general steps in the transformation of cast or pig iron into steel. Various furnaces for use in steelmaking are available, including the electric arc furnaces, the basic oxygen furnace (top blown, bottom blown, top and bottom blown), the AOD furnace, and the open hearth furnace. While choice of the specific furnace to be used and specific procedure to be followed varies based on criteria known to those skilled in the steelmaking art, depending upon the composition, purity and end-use of the steel desired, certain difficulties in steel production exist across these broad procedures.
Primary among the problems faced in steelmaking is the removal of sulfur and inclusions, such as undesirable oxides, silicates and aluminates, from the steel during production. Removal of sulfur is of particular importance and difficulty, as too high a sulfur content deleteriously affects the performance of the resulting steel. Flexibility as to the manner and timing of effecting desulfurization and inclusion removal is also important, yet not readily controllable. Ideally, the steelmaker wishes to be afforded the option to perform desulfurization through treatment of the initial charge in the furnace, before tapping; or through treatment of the steel in the ladle; or through treatment in the tundish.
Of similar importance is the ability to effect the solubilizing in the steel charge of lime and/or other flux materials. Ease in solubilizing fluxing materials in the furnace affects the efficiency, control and speed of the initial melting and heat operations, and impacts on the overall composition, quality and purity of the steel product.
Many solubilizing, desulfurization and inclusion-removing steelmaking additives are known to the art, principal among them being the use of spar, CaF.sub.2. While spar affords the flexibility as to the manner and timing of effecting desulfurization and inclusion removal, its use presents serious drawbacks.
When used to effect desulfurization in the furnace, spar attacks the furnace's refractory lining, particularly eroding the refractory at the slag line. This drastically reduces the number of heats which the lining can withstand before a costly and time-consuming relining of the vessel is necessary. When used to effect desulfurization in the ladle, spar again attacks the refractory lining, necessitating frequent, costly relines and unacceptable downtime for the ladle. Alloy recovery during injection or stirring is also reduced as a result of spar use. A further problem is the excessive heat loss that results from the use of spar in the ladle, as it has poor insulating properties, requiring that the heat be tapped at a high temperature and thereby resulting in a shortening of furnace and ladle refractory lining life. An additional shortcoming is the ineffectiveness of spar to remove inclusions in the ladle, especially during stirring.
When used in the tundish, spar usage results in even greater difficulties. It fails to provide any cover material, resulting in excessive heat loss and a poor temperature profile due to its inferior insulation properties. Use of spar in the tundish also fails to provide effective desulfurization, nor does it remove and retain inclusions. Refractory attack is again a serious problem. Finally, varying degrees of undesired reoxidation and recarburization (if carbon is present) can occur.
It is apparent that there existed in the steelmaking art a serious need for an additive capable of solubilizing fluxing materials, desulfurizing the steelmaking charge, and removing inclusions, such as undesired oxides, aluminates, silicates and sulfides, without presenting the serious shortcomings resulting from use of the known additives, yet providing the steelmaker with the flexibility to effect solubilization of fluxes, desulfurization and/or inclusion removal either in the furnace, ladle or tundish.